1. Field of the Invention
The invention relates to a method and to an apparatus for applying Raman amplification.
2. Description of Related Art
As known, in optical transmission systems, semiconductor lasers or semiconductor laser diodes are often used as light emission devices. Based thereon, the semiconductor lasers or laser diodes can also be used as so-called pump lasers for employing Raman amplification or distributed Raman amplification. Such Raman amplification is increasingly used within high capacity wavelength division multiplex (WDM) systems operating at a high bit rate of at least 10 Gbit/s, in particular exceeding 10 Gbit/s by a multiple and seems to be indispensable in order to improve the system performance, in particular as regards spanned transmission distance.
In this case it is necessary to achieve high optical power level, exceeding a magnitude of 100 mW by a multiple that are pumped inside the respective optical device, such as an optical transmission fibre, a fibre component or any other optical waveguide usable therefore.
For enabling such a Raman amplification the separation in frequency between the data signal to be transmitted and the pump power usually has to be about 13.2 THz. Correspondingly, if the data signal to be transmitted has a wavelength of about 1550 nm, the power of the pump signal should be at a wavelength approximately 110 nm lower than that amplified data signal wavelength.
Currently, Raman amplifiers in use are fibre lasers, using a pump power of up tp 2 W and a spectral width of less than 1 nm, or are so-called fibre Bragg Grating stabilised Fabry-Perot type lasers having a pump power up to 200 mW and a spectral width of less than 2 nm.
However, the spectral distribution of such Raman pump powers is very important. If, for example, the Raman pump provides all the pump power with a high spectral power density such as employed by the aforementioned currently used lasers, in addition to the Raman effects, certain further detrimental non-linear effects are generated. These non-linear effects include, for example, Four Wave Mixing (FWM) and/or a Stimulated Brillouin Scattering (SBS). This results in a reduced efficiency of the Raman amplification or even in degradation of the entire system performance.